1. Field of the Invention
This invention relates to a composite type junction ferrite magnetic head constituted of a single crystal ferrite and a polycrystalline ferrite. More particularly, it relates to an improvement for reducing the slide noise of such a magnetic head.
2. Description of the Prior Art
Recently, for meeting the demand for long playing and miniaturized video tape recorders, for example, and the debut of so-called digital video tape recorders, there have been attempts to make high density recording or a recording at shorter wavelengths. In association therewith, the use of a high coercive force magnetic recording medium has become popular.
With the conventional monolithic ferrite head, it is difficult to achieve good compatibility between the recording and reproducing properties. Thus a so-called junction ferrite type magnetic head has been proposed in which a single crystal ferrite having a high saturation magnetic flux is used at the front gap contacting the magnetic recording medium and a polycrystalline ferrite having a high magnetic permeability is used at the back gap.
This junction ferrite type magnetic head is superior in electromagnetic conversion properties, while being able to make a high density recording, although it has a simplified structure similar to that of the monolithic ferrite head.
In preparing the aforementioned junction ferrite magnetic head, one may use a glass melt bonding method for bonding the single crystal and polycrystalline portions to each other. Although it is possible with this method to set the melt bonding temperature to as low as 900.degree. C. to reduce the diffusion of the ferrite monocrystals, this method has the disadvantages that a glass film with a thickness of the order of 300 to 1200 .ANG. is interposed between the single crystal ferrite and the polycrystalline ferrite, so that an internal strain is produced due to the difference in thermal expansion coefficients; that the magnetic reluctance is increased due to the presence of the insulating glass material so that the S/N ratio is lowered; and that the glass tends to erode the ferrite portions of the magnetic head.
There has also been proposed a so-called hot press working process according to which the single crystal and polycrystalline ferrites are directly bonded together under a nitrogen atmosphere at a pressure of not lower than 5 kg/cm.sup.2 and a temperature of 1200 to 1300.degree. C.
However, proper selection of the bonding conditions is difficult to achieve with this hot press working process. Moreover, the interface between the single crystal and polycrystalline portions tends to be inundated due to dispersion of the ferrite monocrystals. In addition, since the diffusion layer at the junction interface between the single crystal ferrite and the polycrystalline ferrite may reach 70 to 300 microns, considerable dispersons may be produced in the thickness of the single crystal ferrite portion of the produced magnetic head.
On the other hand, our experiments have revealed that, when the aforementioned junction ferrite is used in the magnetic head for a video tape recorder, the thickness of the single crystal ferrite portion, including that of the diffusion layer, is closely related with the sliding noise, such that, as the single crystal ferrite portion becomes larger in thickness, the sliding noise is concomitantly increased.
Therefore, while the thickness of the single crystal ferrite portion should ideally be about equal to the depth of the magnetic gap, such thickness is difficult to control due to diffusions or inundations in the interface when the magnetic head is produced by the aforementioned hot press working. Thus the current practice is to design the thickness so as to be slightly larger than the depth of the magnetic gap for the sake of safety.
The technique of providing Co, CoO or Co.sub.3 O.sub.4 in the interface has also been proposed, but the noise level of the magnetic head may not be lowered since the diffusion of the monocrystal ferrite may inevitably be of the order of 100 microns.